Circuit breakers have switching contact devices for interrupting in each case a line connecting two line terminals for a (phase) line in the circuit breaker. In the case of three phase lines, there are also three switching contact devices. In the circuit breakers, there is a triggering device which can act mechanically on the switching contact devices. In the present case, the triggering device is intended to operate digitally, that is to say comprises a data processing device to which input signals are supplied. The input signals are measuring devices for measuring the current intensity of a current flowing through the lines between the line terminals. For this purpose, current transducers are provided in the circuit breakers for each phase line or the associated inner line in the circuit breaker, in the form of Rogowski transducers, coils without iron core, typically having a plastic core. The input signals generated by such Rogowski transducers are supplied to the triggering device and this is designed for driving the switching contact devices in dependence on these input signals.
Apart from the three phase conductors (or sometimes only one), there is always also a neutral conductor. Normally, no separate current transducer is provided for this.
A circuit breaker may be equipped with one or three switching contact devices and in each case a current transducer for these but does not have a switching contact device for the neutral conductor and no current transducer either. However, it should be possible to upgrade it. In this case, the circuit breaker comprises an external main terminal to which a current transducer which does not belong to the circuit breaker can be connected; thinking in this case of a current transducer, especially a Rogowski transducer, which surrounds the neutral conductor. The input signals originating from this current transducer, which is connected externally, are also intended to be supplied to the triggering device but, instead of a separate switching contact for the neutral conductor, the switching contact devices for the phase lines are driven. It is thus possible to upgrade a so-called three-pole circuit breaker, that is to say a circuit breaker without monitoring of the neutral conductor, to become a four-pole circuit breaker.
However, the facts of the matter are that it is a mark of upgradability that an external Rogowski transducer is not always connected.
Different from current transducers having an iron core, the measuring inputs of the circuit breaker must have a high impedance for the case of the Rogowski transducer being connected. In this way, however, the measuring inputs, that is to say the sensor terminals of the current measuring device, are highly susceptible for interference signals being coupled in, especially when no Rogowski transducer happens to be connected. In this case, the measuring input would be operated with both sensor terminals open, and especially in the case of the so-called ground fault detection, possible interference signals could lead to a mistriggering of the circuit breaker, that is to say to an opening of the switching contacts when this is not indicated at all.